Annual Floods as a Key to Fertility (SSAWRN)

Alan Anderson

Gaolathe Tsheboeng, congenial and easygoing, had a smoother and more direct path into the RISE program than those who spent years searching for financial support while surviving on less stimulating work. Gaolathe, living in Gaborone, the capital of Botswana, heard about RISE just as he was finishing his BS degree at the University of Botswana and was quickly able to take advantage of it. He also differs from most of his peers in having earned a degree in education, rather than in science, though he did take a science emphasis. He has had experience teaching “integrated science” in public school, but the RISE scholarship has given him a chance to move into an experience of hands-on research – in an area of high importance to Botswana.

Like most of his peers at the Okavango Research Institute (ORI), his research is part of a broad effort to understand the relationships between the annual flooding, the vegetation, and the animals of the Okavango Delta. Gaolathe came to the ORI at a fortuitous time to study these relationships. A two-decade-long drought had just been swept away by the second-largest flood since record-keeping began in 1923, offering a dramatic contrast of conditions. When he was asked to study the relationship between flooding and vegetation distribution, he was able to drawn on the results of two studies done during the drought, in 1996 and 2004, and compare them with the current flood of 2010. Last March he began visiting the same areas studied earlier, comparing the levels of nutrients and kinds of plants found there during drought with those found during heavy flood times.

The primary plant in his study was the abundant sedge Cyperus esculuntus, of the same genus as the papyrus cultivated by early Egyptians to make writing material. One of the results he found was predictable: The Cyperus, accustomed to growing in wet areas, had now invaded areas that had been dry for many years but were now wet. At the same time, the plants that had been growing in these newly wet areas had disappeared.

But another result was a surprise – one that may have important policy implications for the Delta. While the ORI scientists expected the large flood of 2010 to reduce the concentration of nutrients in the water by a process of dilution, in fact the nutrient levels were found to be higher than they were during years of low flood. The hypothesis of Gaolathe and his colleagues is that the nutrients accumulated from years of the build-up of dead plants and animal dung were released by the fresh flow of water and gave rise to sudden increases in plankton, fish, and bird populations. “We think that when more area is covered by the flood, more nutrients are pulled from those areas. We also assume that there was burning of plants upstream, and the big flood brought down the nutrient-rich ashes to our flood plain.”

A logical consequence of this nutrient cycling is that the flooding is necessary to preserve the natural richness of the Okavango Delta and its biota. Gaolathe and others hypothesize that any major hydrologic changes that reduce or harness the flooding – such as construction of dams upstream, which has long been contemplated by Angola – would reduce the rich plant and animal biodiversity of the ecosystem. This, in turn, would threaten the native cattle herders, fishermen, and flood recession farmers, as well as the large tourist industry on which the nation depends.

See update here.